In many situations, it is desirable to identify and monitor the status of multiple objects that are in proximity with one another. For example, in a commercial or industrial facility such as a factory or a warehouse, multiple sensors, motors, or other devices can be positioned at different locations around the facility. In such an environment, it can be desirable to monitor which of these devices are present and, if desired, which devices are operating properly, as well as to receive and transmit information from and to these devices. Similarly, in a hospital environment, it can be desirable to monitor and communicate information with numerous devices such as patient monitors, analytical devices, food carts, blood/urine samples, etc.
The use of wireless communications in any environment makes it possible for individual objects of interest to be added to, or removed from, the larger group of objects without incurring significant costs associated with installing or removing a fixed communications media. Conventional systems for providing such wireless communications employ a central transceiver (interrogator) that is in communication with the transponders. The transponders, which commonly are referred to as radio frequency identification (or “RFID”) transponders (or “tags”), are capable of receiving an inquiry signal from the central transceiver and providing information back to the central transceiver in response to those inquiries.
Often the amount of information that must be communicated by way of such systems is fairly limited. For example, it may be the case that the only information that must be communicated from the objects of interest to the central monitoring terminal is information indicating the identity, presence, or on/off status of the objects of interest. Nevertheless, conventional systems for providing such communications require significant amounts of energy to be transmitted over long distances. Also, data collisions with multiple transponders can occur with only a few transponders present. Classical methods also require either significant bandwidth or very long read times when many tags are present.
In particular, the RFID transponders in conventional systems typically send information to the transceiver by way of signals employing Amplitude Shift Keying (ASK) or Frequency Modulation (FM). Because these signal formats are employed, the information signals being sent by the transponders to the transceiver are “on” (e.g., nonzero signal levels are being transmitted) from about 50% to 100% of the overall time period over which the ASK signals are being sent. Such continuous or nearly-continuous transmission signals require large amounts of energy which limits the transmission range of the system. Further, as the proportion of on time reaches such high levels, the probability of collisions between (e.g., interference among) information signals originating from different transponders becomes high as the number of transponders in communication with the transceiver increases.
In view of these limitations of conventional systems, it would be advantageous if a new system providing communications between multiple transponders and a central monitoring terminal could be designed that was more efficient than conventional systems, both in terms of the system's need for large numbers of transponders and in terms of lowering the system's demand for energy.